Generally, there are screw-type and plunger-type injection devices. As representative examples, a screw-type injection device is disclosed in the patent literature 1 and a plunger-type injection device is disclosed in the patent literature 2. As disclosed in these literatures, the injection device is mainly formed of a cylinder and a screw. Pellets are inserted from a hopper provided in the cylinder and are conveyed to the injection nozzle side by rotation of the screw inside the cylinder, while they are heated and melted. Then, the melted resin is gathered at the tip end of the nozzle and injected so that the melted resin is conveyed to a die.
Generally, plastic pellets (hereinafter referred simply as “pellets”) are made from plastic (synthetic resin), and the heat conductivity is from 0.07 to 0.20 kcal/m·hr·° C. This is from one several hundredth to one several thousandth of the heat conductivity of metal. In view of this, the pellets are approximately heat insulating material. Accordingly, even if the pellets are given a sufficient amount of melting heat to be melted, the heat is hard to reach the inside of the pellets (centers of the pellets), and it takes much time to heat the pellets completely.
Therefore, it is difficult to shorten the time from when each pellet is melted sufficiently and to when resin molding is produced. The pellets need to be melted for a relatively long time in the cylinder and the working efficiency is not good. Besides, in the injection device, each solid of the many inserted pellets in the cylinder is heated and moved to the injection side by rotation of the screw, and at this time, a part of many pellets is pressed against the inner wall of the cylinder.
In other words, the pellets are pressed by the inner wall of the cylinder. Then, the surface of the solid of each pressed pellet is partially in contact with the inner wall of the cylinder and melting of each pellet is restricted to a part of the pellet solid in contact with the cylinder. The pellets kneaded in the cylinder by the screw are moved away from the inner wall of the cylinder for a short time and therefore, the pellets are not heated sufficiently and pellet solids are not melted as a whole, resulting in mixture of melted parts and non-melted parts of the pellets.
As the pellets are pressed against the inner wall of the cylinder by screw in a repeated manner, the pellets are completely melted, and when melted pellets are conveyed toward the nozzle, the amount of resin pooled in the cylinder is equal to or more than several tens of times of the amount that is needed for one injection so that an unnecessary amount of pellets remains in the cylinder.
Besides, when the melted resin passes through a gap between the screw and the cylinder, the resin suffers mechanical damage. Particularly, such a problem is often caused when melting pellets with glass fibers and the screw may be worn. Further, as each pellet is partially melted at random, it is inevitable that the like pellets may remain everlastingly in the cylinder. Accordingly, the operation may become particularly difficult when changing pellet materials in the cylinder.
As a substitute as such a screw type, there is a plunger-type injection device. Such a plunger type is simple in structure and is likely to be downsized. For the plunger type, it does not have the defect that the screw may be worn. The patent literature 2 discloses the plunger type injection device having the most basic structure, which is configured to have a head-cut conical shaped heating cylinder having a plurality of through holes, an injection plunger, a supply tube and so on. The injection plunger is used to convey a synthetic resin material to the heating cylinder to be injected. However, the injection device of the patent literature 2 also has various problems.
In the patent literature 2, the injection plunger and the head-cut conical shaped heating cylinder are formed to have different diameters at their facing surfaces, and the diameter of the injection plunger is formed one size smaller than the diameter of the heating cylinder at the facing portion. In addition, there is formed an air space surrounded by the tip end of the injection plunger, the heating cylinder, a tip end part of the injection plunger and a supply tube, and this air space is a larger area than the area at the tip end of the injection plunger.
Accordingly, the melted synthetic resin material is once pushed into the air space by the injection plunger, but even when the injection plunger further moves toward the heating cylinder, the synthetic resin material is not able to flow into the through holes of the heating cylinder efficiently and some material may remain in the air space without flowing into the heating cylinder. Then, the residual synthetic resin material in the air space may become an obstacle to a synthetic resin material that is to be newly supplied into the through holes of the heating cylinder. And, there may arise a problem of mixture of the synthetic resin material to be newly supplied and residual resin remaining for a long time to be deteriorated.
Then, the applicant of the present application has developed an injection device in a molding machine which is capable of eliminating the inconveniences of the injection plunger and the head-cut conical shaped heating cylinder and performing the pellet resin melting step and the melted resin injection step extremely efficiently, as disclosed in the patent literature 3. According to this patent literature 3, there has been provided an epoch-making invention that only by pressing a lump of pellets by the plunger, it is possible to melt the pellets into melted resin and inject the melted resin simultaneously.
According to this invention, in the melting step of melting pellets, the pellets pass through a plurality of cone-shaped holes of a heated melting device with a predetermined pressure and when the pellets go out of the outlet, the solid pellets become melted resin. That is, the melting speed and the injection speed are set to be the same in consideration of the quality of pellet material, viscosity of melted resin, melting temperature, pressure, melting speed, injection speed, flow rate and so on. With this invention, melting is very accelerated in terms of the melting speed, but then, in terms of the injection speed, injection takes the same time as the melting and the injection is felt slow.